The reason is power supply of mobile phone is a battery bank, and it is very limited,if uplink frequency is more,i.e. frequency of wave from mobile phone to telecom tower is more, then large amount of power is needed to transfer this frequency,so more power should be consumed in mobile phone so user have to charge his mobile phone after a very less interval which must be annoying for users,after noticing this fact designers made a system in which uplink frequency is less than downlink frequency.

NOTE: In satellite system uplink frequency is more than downlink frequency.